<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <ul>

        <li>1</li>
        <li>2</li>
        <li>3</li>

    </ul>
    <script>
        const ul = document.querySelector('ul')
        //1、克隆节点  元素.cloneNode(true)
        const li1 = ul.children[0].cloneNode(true);
        // console.log(li1);
        //2、追加
        ul.appendChild(ul.children[0].cloneNode(true))
        ul.appendChild(ul.children[0].cloneNode())//内容是空的

    </script>
</body>

</html>